As many as 7.5 million students nationwide miss a month of school each year, with absenteeism highest in kindergarten and in high school, according to Johns Hopkins University researchers.

An estimated 10 percent to 15 percent of students are "chronically absent" from school and miss enough class time to be considered a "severe risk" of not finishing high school. In urban and rural areas, where students may be poor and come from families with little education, the figures are as high as one-third of students, the study shows.

In Maryland, chronic absentee rates for poor students were "more than 30 percent, compared to less than 12 percent for students from more affluent families," according to the research.

"Because we don’t measure or monitor the problem, we generally don’t act on it," Robert Balfanz, director of the Everyone Graduates Center at Johns Hopkins, said in a statement. "Left untreated, the problem will likely worsen achievement gaps between rich districts and poor districts and curtail the positive effects of promising current and future reforms."

Balfanz is one of the Johns Hopkins researchers who worked on the study. Vaughan Byrnes, a research associate, co-authored the report.

According to the study, Balfanz says only six states track chronic absenteeism: Georgia, Florida, Maryland, Nebraska, Oregon and Rhode Island. Several states, including California and New York, do not gather individual data needed to calculate chronic absenteeism, the study found.

The U.S. Department of Education doesn't track the problem; it only requires states to monitor daily attendance, according to the study.

A student is considered chronically absent if he or she misses at least a month of school in a year, or about 10 percent of school days.

Because of the limited data, Balfanz said the study was only an "educated guess" about the scope of the problem.

"Even so, the research shows that we must address the attendance problem if we're going to have the kind of broader school improvement we want and our students deserve," said Marie Groark, executive director of Get Schooled, a nonprofit group that funded the study. "When you think about it, missing one or two days a month doesn't seem like much but it adds up over time. Then, that becomes a problem and that problem has a consequence."

The Get Schooled Foundation is an educational nonprofit campaign focused on boosting graduation rates. It is partially funded by the Bill and Melinda Gates Foundation.